This invention relates to development systems for electrostatic processors and, more particularly, to methods and means for reducing the routine maintenance required by those systems.
In a conventional electrostatic printing process of the type described in Carlson's U.S. Pat. No. 2,297,691 on "Electrophotography", a uniformly charged photoreceptor is selectively discharged in an image configuration to provide a latent electrostatic image which is then developed through the application of a finely divided, resinous material, called "toner". As is known, that process has enjoyed outstanding commercial success, especially in plain paper copiers and duplicators. Nevertheless, substantial effort and expense are still being devoted to the perfection of the process, including the development step.
The vehicle normally used in electrostatic processors to deliver the toner is a multi-component developer comprising toner particles and relatively coarse "carrier" particles. The toner and carrier (or sometimes carrier coating) are formed from materials which are removed from each other in the triboelectric series, thereby enabling a triboelectric charging process to be employed to induce electrical charges of opposite polarities on the toner and carrier particles. The polarity of the charge for the toner particles is selected to oppose the charge of the latent image so that there are competing electrostatic forces acting on those particles. Specifically, the toner particles at least initially tend to be attracted to the carrier particles, but are subject to being electrostatically stripped therefrom whenever the developer is brought into the immediate proximity of or actual contact with an image bearing photoconductor.
Provision is commonly made in existing development systems for adding additional toner to the developer from time-to-time so that the toner concentration remains at a suitably high level. However, there still is a recurring problem stemming from the limited useful life of available developers -- viz., users of electrostatic processors are faced with the messy and time consuming, routine maintenance task of purging the development system of exhausted developer and recharging it with fresh developer. If that is not done on schedule, there is a marked reduction in the quality of the copies produced.